Stable-surface iron-chromium-silicon alloy



' Patented Oct. 2, 1928.

UNITED STATES PATENT OFFEQE;

PERCY E. ARMSTRONG,-OF LOUDONVILLE, NEW YORK, ASSIGNOB TO LUDLUM STEEL COMPANY, OF WATERVLIET, NEW YORK, A CORPORATION OF NEW JERSEY.

STABLE-SURFACE IRON-GHROMIUM-SILICON ALLOY.

No Drawing.

My invention 'relates to a stable surface iron-chromiumsilicon alloy, especially adapted to the production of rolled products,

I have found that within of about 1330% as for example, rolled bars, plates and sheets.

In the ferrous alloy of the present invention the carbon is kept as low as practicable and preferably under .05%, the presence of carbon being detrimental to the stable surface qualties and also to the workability of the material.

The-chromium of the alloy should be sufficiently high to give good working qualities. A peculiar softening efiect appears to set in with the chromium-iron alloys with such low carbon, at about 13% of chromium. At about 16.5% chromium this softening effect seems to be as great as with still higher chromium. Higher chromium is, of course, expensive and does not appear to add materially to the stable surface qualities.

herefore, while I may use up to about 30% chromium, which is about as high a per centage as will be practically utilized, I prefer not to go over about 20% chromium. the broader range chromium a narrower range of about 16-20% chromium can be used to great advantage and a still narrower range of about 16.5-18.5% chromium is preferable for securing desirable softness and workability with the requisite stable surface qualities.

The silicon adds materially to the stable surface qualities, especially when the material is to be exposed to hot oxidizing gases, and also adds to the hot workability, the combination, essentially of iron, chromium and silicon, having the characteristics for rolling, which are found in a pure metal made up of a single element. While the effects of silicon are to be noted with silicon as low as .30%, higher silicon of about .5-5% is preferably used. Silicon higher than about 2.5% is not desirable in large cross-section bars or heavy rolled plate because of a tendency to resulting brittleness at atmospheric temperatures.

The principal part of the remainder is iron, though small percentages, preferably not to exceed 5% each, of other metals may be present to give the properties normally resulting from such additions. For example, small additions of tungsten, molybdenum, manganese and nickel have a desirable toughening efiect upon the alloy, where- Application filed February 19, 1924. Serial No; 693,920.

as larger additions of tungsten' and manganese do not seem to convey any great benefit to the alloy. Nickel, however, ,in large perchinability; and manganese is particularly desirable up to about .50% as it adds slightly to the rollability at rolling mill temperatures; whereas higher manganese does not seem to add anything to hot malleability of the alloy.

The preferred method of making this alloy is by heating substantially carbon free. or low carbon chromium or ferrochrome to a fairly high heat (the higher the temper ature the better, so long as it is below the melting point) and adding it to or adding to it, practically carbon free molten iron with suitable'additions of substantially carbon free ferrosilicon or metallic silicon, which is also preferably not molten. It is essential'in order to procure sound ingots or castings that the iron be degasified, preferably by aluminum, before the alloy addition is made, Instead of .or adjunctive to making the silicon introhighly heated, but i duction separately, 'ferrochro'me made by silicon reduction may be used containing suitable proportions of silicon. Where the heated chromium material is charged into molten iron the slag on the latter aflords ample slag for the former, but when the latter is added to the former, neutral slag may advantageously be thrown over the hot chromium material, or a slag heated up with the chromium material before making the iron addition. When adding manganese, tungsten, or nickel, or any other alloying material, same can be conveniently mixed with the chromium and heated up therewith prior to adding it to or adding to it the molten substantially carbon free iron. This, however,

is not a prime requisite because the percentder about 1550 F.,

dust and to furnace, etc., linings, and the about 1% silicon and carbon about 03% is particularly well adapted for pack rolling into sheets. When pack rolling is resorted to, particularly for thin gauges, it is advisable to first give to the sheet a charcoal or similar wash, which gives a very good finish on the pack side after rolling.

Stable surface alloy of analysis within my limits cannot be hardened to any substantial extent by heating and quenching. For example, an alloy containing chromium about 17%, silicon about 1%, carbon about 03%, manganese about .4% and the balance principally iron, when fully annealed has about 160 Brinell hardness, the same alloy as rolled or as air cooled is-about 170 Brinell hard; quenched from temperatures under 1900 F. about 170; and quenched from above about 1900 F. the Brinell hardness figure is about In commercial and quantity production,

- the chromium-iron-silicon alloys within my limits are preferably hot finish rolled. During hot rolling, the material should be subjected to as much mechanical working as it is capable of withstanding. The toughness of the material to impact is greatly increased in hot rolling by the use of heavy drafts at roll passes. It is undesirable when rolling from initial high rolling temperatures to use relatively light drafts at roll passes, such as are generally used in rolling alloy steels such as high-grade tool steels, for example. With rolling mill equipment capable of producing the desirably great reduction at roll passes, the alloy may advantageously be heated up to fairly high initial temperatures, for example, around 1900- 2300 F. for the hot rolling. The finishing temperature for this rolling should be unas with higher rolling finishing temperatures, the tendency is to produce brittleness, which is apparently due to large grain size. This hot rolling with heavy passes may be advantageously finished at a temperature in the neighborhood of 15001400 F., or somewhat lower.

If desired, and especially if the mill equipment available for rolling the alloy is not capable of producing sufficient reduction between passes to obtain agood degree of toughness in the manner just described, then after rolling to about 1200-1300 such high initial rolling heats should be avoided, or, in other words, the initial rolling temperature to which the alloy material is heated should be kept low. For example, with such relatively light draft rolling from relatively low initial temperatures the billets, blooms, sheet bars and the like for final rolling should be heated to a temperature in the neighborhood of 1500-1600 F., depending somewhat upon the amount of' total reduction to be given, and the finishing temperature should be under about 1400 F.. and preferably under 1100-1200 F. The alloy has good hot malleability at about 1000 F., for example, and the rolling can be carried down. to atmospheric temperatures, if desired.

Quite different the two described methods of working. The fracture of the alloy rolled at the hightemperaturcs passes and the rolling finished at about 1450 F. or somewhat higher is substantially crystalline with a tendency toward lamination when, a broken surface is visually observed without the microscope. If in this hot rolling heavy work method the temperature of the finishing of the rolling step is carried low, say, for example, down to about 1300- 1100 F. or lower, the structure becomes more noticeably lamellar, this tendency being increased as the finishing temperature is lowered. The material in either case is quite tough, but still greater toughness to impact can be produced 'byreheating the bar, plate, sheet or the like after finish rolling to a temperature where a new grain birth (not an accompaniment of passing through a critical temperature, form of release of mechanical stresses) is discernible, the laminations being practically obliterated, and the material is extremely tough. For example, in the case of such hot rolled products rolled at high temperatures using heavy drafts between passes, with or without the low temperature finishing rolling, they may thus be reheated after finish rolling to about 15001600 F. and quenched in oil or water or. allowed to-v cool in the atmosphere on very slowly cooled, with resulting increase in toughness and in their notched bar impact value.

If, however, low initial temperatures and relatively light passes have been employed in finish rolling, the fracture is substantially entirely lamellar in its nature, and it is undesirable to heat treat the alloy after rolling in such a way that this lamellar structure is substantially broken up, since, if this be done, it is brittle to impact and this is apparently due to larger size crystal structure and intergranular weakness. The material when rolled at low temperatures and with relatively light passes is best reheated F., and

fractures are produced by but apparently a with the heavy reductions between quenched in oil, or water, cooled in theair advantage in finish rolling, but may beused to advantage from the ingot to the finished hot worked product. The alloy may be forged instead of rolled, and in forging the above stated relation of temperatures and vigor of working should be observed.

I claim:

1. The process of rolling stable surface alloys containing principally chromium, silicon and iron, with chromium over about 13%, silicon about :3-5%, with carbon as low as practicable and the principal part of,

the remainder iron, which comprises heating to a comparatively high rolling temperature and rolling same with substantially maximum draft roll passes.

2. The process of rolling stable surface alloys containing principally chromium, silicon and iron, with chromium over about 13%, silicon about .35%, with carbon as low as practicable and the principal part of the remainder iron, which comprises heating stantially maximum draft roll passes, and finishing the rolling at temperatures of about ll00-1500 F.

3. The process of rolling stable surface alloys containing principally chromium, silicon and iron, with-chromium over about 13%, silicon about .35%, with carbon as low as practicable and the principal part of the remainder iron, which comprises heating to about 1900-2300 F., rolling with substantially maximum draft roll passes, finishing the rolling at temperatures of about 14001500 F., and reheating to about 15001600 F. and cooling;

4:. The process of making wrought articles of alloys containing principally chromium, silicon and iron with chromium over about 13%, silicon about .35%, carbon as low as practicable and under .05%, which comprises putting together degasified molten iron and preheated chrome alloy material, casting into ingots, hot working in'to reduced sizes, heating to an initial rolling temperature of about 1900-2300 F., and finish rolling with substantially maximum' draft roll passes, finishing the rolling at a temperature above about 1100 F.

In testimony whereof, I havesigned my name hereto.

PERCYA. E. ARMSTRONG.

to" about 19002300 F., rolling with sub- 

